The present invention relates to a process for the removal of ammonia from wastewater and more particularly, to a process wherein ammonium ions are adsorbed onto zeolite and subsequently recovered as ammonia gas.
The presence of ammonia in wastewater is undesirable since the nitrogen contained therein acts as a fertilizer for aquatic plant life thereby resulting in the phenomenon commonly known as "algae bloom". Heretofore, methods have been proposed for removing ammonia from wastewater. One such method involves liming of a body of water which converts the ammonium ions to gaseous ammonia which is then liberated, via air stripping into the atmosphere. Rather than eliminate the adverse environmental effects of ammonia however, liming merely transfers the problem from the water to the atmosphere. Additionally, since useful products may be produced from ammonia as a starting reagent, the liberation of ammonia into the atmosphere represents a significant economic loss.
In U.S. Pat. No. 4,011,156, a method of eliminating organic and inorganic bound nitrogen from domestic and industrial wastewater is disclosed. The process is generally carried out by passing the wastewater, an alkaline substance, and oxygen through a fixed bed reactor containing a catalyst whereby the ammonia is converted to harmless nitrogen gas. The alkaline substance is added since a marked decrease in pH occurs as the NH.sub.4.sup.+ is oxidized to N.sub.2 and since such a pH decrease was observed to adversely affect efficiency of the reaction as well as the catalyst.
While the conversion of wastewater ammonia to nitrogen gas is highly desirable from an environmental viewpoint, it is not very economical since a potentially valuable material is being converted into a useless form and expelled into the atmosphere. Additionally, the process itself is cumbersome, requiring a practitioner to evaluate the pH values of the reaction mixture along different points of the fixed bed.
In U.S. Pat. No. 3,984,313, sewage water containing ammonia as well as phosphates is purified by using a ferric hydroxide loaded ion exchange resin. While the proposed process is desirable economically since ammonium hydroxide, which is a commercially useful end product, is produced from the ammonia contaminant, the process itself suffers from the disadvantage that it is not continuous since the wastewater feed must be temporarily shut down and the spent resin replaced with fresh resin.
It has been observed that zeolites have a high affinity for ammonium ions and would therefore be quite effective as adsorbents for removing the ions from wastewater solutions. Since zeolites have no affinity for ammonia gas, the ammonium ions initially adsorbed onto the zeolite could be desorbed merely by increasing the pH enough to convert the bound ammonium ions to unbound gaseous ammonia. Such a process would prove highly desirable from both an economic and environmental viewpoint since the bulk of the ammonium ions would be removed from the wastewater and since the ammonia subsequently released is not only a commercially valuable material as is, but also may be converted relatively easily to a host of other commercially valuable materials such as ammonium phosphate.
Despite their high affinity for ammonium ions as well as the environmental and economic advantages discussed above, zeolites have not proven completely effective when used in conjunction with conventional purification techniques such as fixed or pulsed beds. One reason for this lack of effectiveness is the low loading capacity of zeolites, i.e., the amount of ammonium ion which can be adsorbed per unit of zeolite. Thus, in order to effectively use zeolites as adsorbents in conjunction with ammonia removal processes, an inordinate amount of zeolite would be required thus resulting in increased costs in terms of materials, equipment, energy expenditures, and down-time. Such drawbacks would be in addition to those already described with respect to prior art fixed bed processes utilizing other adsorbents or ion exchange resins. Such drawbacks include difficulties in carrying out the reaction continuously as well as difficulties in maintaining the optimum pH as material is reacted or adsorbed.